Wireless local area networks have become prevalent with the standardization of the IEEE 802.11 family of protocols. In particular, the IEEE 802.11b standard, offering data rates up to 11 Mbps, has been widely adopted by numerous access points and network interface card manufacturers. These networks use the wireless medium in the 2.4 GHz ISM (Industrial, Scientific and Medical) band as the transmission channel.
Spread spectrum communications provide efficient utilization of signal bandwidth and power. An advantage of a spread spectrum communication system is its ability to reject interference whether it is unintentional interference by another user simultaneously attempting to transmit through the channel, or the intentional interference by a hostile transmitter attempting to jam the transmission. Spread spectrum communications also provide excellent narrow-band noise rejection characteristics.
Spread spectrum communication systems spread a baseband digital signal with a periodic binary sequence, noise-like in nature, called a pseudo random noise (PN) sequence. Through this spreading technique, the relatively narrow-band digital baseband signal is made to appear as wide band noise. Furthermore, the receiver knows the pseudo random noise sequence used by the transmitter in order to properly recover the transmitted signal. Any other additional receiver listening on the channel will not be capable of recovering the transmitted message without the correct pseudo random noise sequence, hence the secure nature of this type of communication.
The same pseudo random noise sequence used to de-spread the received signal, that is, convert the wide band signal to a narrow band signal, will spread any narrow band noise, such as jamming signals, to a wide band signal. In effect, this makes narrow band noise appear as wideband noise at the receiver input, improving performance.
In a Spread Spectrum communications system, a pseudo random noise sequence is used to convert a narrow-band digital signal to a larger bandwidth signal, referred to as a spread signal. To transmit the spread signal through a channel such as air, the signal is modulated and mixed with a sinusoidal carrier to translate it to the appropriate frequency band.
Synchronization is of concern with the recovery of the baseband digital signal. For proper operation, a spread spectrum system requires that the locally generated pseudo random noise sequence used to de-spread the received signal be synchronized to the pseudo random noise sequence used to spread the transmitted signal.
When a locally generated pseudo random noise sequence is compared to an interval of the received signal, a measure of correlation is used to determine when the two signals are satisfactorily aligned. After alignment, the remaining received signal is then correlated with the pseudo random noise sequence and the received signal is properly de-spread using a matched filter and the baseband digital data is properly recovered.
The single sided spectral occupancy of a baseband 802.11b compliant signal is restricted to within 11 MHz. Traditional 802.11b compliant systems have used a 22 MHz (2×) sampling rate to capture the baseband spectrum using an Analog to Digital Converter (ADC). The output of the ADC is passed on to a digital baseband processor that implements timing recovery, equalization and demodulation.
Traditionally, 802.11b receivers have been narrow band. This implies that the RF (Radio Frequency) frontend and the ADC grab only as much spectrum as is required to process a single 802.11b channel. The evolution of higher sampling rate and higher precision ADCs has enabled a new class of digital baseband architectures that can process multiple channels simultaneously. A problem associated with such architectures is how to determine optimum sampling points in the received baseband signal.